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Assessing Dosimetric Effects On Rejected Head and Neck Kilo-VoltageCone-Beam CTs

M Andrews

M Andrews1*, S Lin2 , N Joshi1 , S Koyfman1 , E Murray1 , N Yu1 , P Xia1 (1) Cleveland Clinic Foundation, Cleveland, OH (2) Cleveland State University, Cleveland, OH


WE-RAM2-GePD-J(A)-1 (Wednesday, August 2, 2017) 10:00 AM - 10:30 AM Room: Joint Imaging-Therapy ePoster Lounge - A

Purpose: Kilovoltage cone-beam CT (kV-CBCT) is frequently used to verify daily setup accuracy for HN patients. Based on daily image review, some kV-CBCT alignments were rejected by physicians. The purpose of this study is to determine whether there is a clinically efficient method to assess dosimetric effects of rejected kV-CBCTs.

Methods: Twenty-nine rejected and 29 approved kV-CBCTs from HN patients were assessed with two methods. For method 1, kV-CBCTs were fused with planning CTs and contours were manually transferred from planning CT to the corresponding kV-CBCT. Dosimetry was calculated using the treated isocenter. For method 2, kV-CBCTs were rigidly registered with planning CTs using Philips’s dynamic planning system (DP) to transfer contours and treatment isocenter. Dose calculations were performed without heterogeneity corrections on kV-CBCT and original plans were recalculated without heterogeneity correction for comparison. Endpoints recorded were doses received by 95% of the GTV and high-dose CTV, and maximum point doses (D0.03cc) to the spinal cord and brainstem. For each method, shifts between registration and acquisition isocenters were compared to clinical shifts.

Results: No difference in dosimetric endpoints were found between accepted and rejected kV-CBCTs. Percent change in spinal cord and brainstem D0.03cc measurements showed significant differences for method 1 vs method 2 for both groups. With method 2, significant differences in calculated (-0.31 ± 0.30 cm) vs. clinical shifts (-0.15 ± 0.23) were found in the AP direction for both groups. Clinical shifts differed significantly between accepted (0.12 ± 0.23) and rejected (0.28 ± 0.23) kV-CBCTs in the SI direction. Calculated shifts significantly differed in the AP direction for accepted (-0.09 ± 0.30) vs rejected (-0.31 ± 0.30) kV-CBCTs.

Conclusion: Daily review of kV-CBCTs is recommended and attempts to estimate dosimetric effects should account for clinical shifts to ensure accurate dose information.

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